This invention relates to optical glass fibers, and, more particularly, to the preparation of optical glass preforms of fluoride glass and to optical fibers drawn therefrom.
Optical fibers are strands of glass fiber processed so that light beams transmitted therethrough are subjected to total internal reflection. A large fraction of the incident intensity of light directed into the fiber is received at the other end of the fiber, even though the fiber may be hundreds of meters long. Optical fibers have shown great promise in communcations and other applications, because a high density of information may be carried along the fiber and because the quality of the signal is less subject to external interferences of various types than are electrical signals carried on metallic wires. Moreover, the glass fibers are light in weight.
Optical glass fibers are typically fabricated by preparing a preform of glasses of two different optical indices of refraction, the glass of higher index inside the other, and then processing the preform to a fiber by drawing or extruding. The optical fiber is coated with a polymer layer termed a buffer coating to protect the glass fiber from damage during later use. The resulting optical fiber has a core of the glass of higher index of refraction, a casing of the glass of lower index of refraction, and the overlying buffer coating. Light is transmitted through the core over great distances with little loss of energy, because the light is subject to total internal reflection at the core/casing interface due to the differences in the indices of refraction.
The optical fiber is made from two glass materials, in the manner indicated, that are selected to have the required optical properties for particular applications. As commonly used, the term "glass" refers to materials that are transparent to radiation such as visible light, so that they permit radiation energy to pass or conduct the radiation, but prevent passage of matter. The radiation may be visible light, but can also include those forms of radiation that are not visible to the human eye. For example, infrared energy, having a wavelength greater than that of visible light, is not visible to the human eye. Infrared light includes electromagnetic radiation having wavelengths of from about 0.8 to about 8 micrometers, and sometimes beyond. Infrared light is used in a variety of devices, including fiber optic communications systems, detectors, photocells, vidicons, and the like.
Optical fibers for visible light are made of silicon dioxide based glasses. These glasses are readily prepared and are highly transmissive to visible light having wavelengths of from about 0.3 to about 0.7 micrometers, and to certain other forms of electromagnetic radiation. However, the silicon dioxide glasses have much poorer transmission of infrared energy, and generally cannot be used as optical fibers for transmission of infrared energy having a wavelength greater than about 1.8 micrometers, except over very short distances.
Glass compositions based upon metal upon metal fluorides are known to have good transmission to infrared radiation, and have been successfully tested for use in infrared fiber optical systems. However, the techniques used to fabricate optical fiber preforms of silicon dioxide glasses cannot be readily used to fabricate corresponding preforms of metal fluoride glasses, and other approaches must be developed.
A preferred method for fabricating preforms of silicon dioxide glasses is by chemical vapor deposition, wherein two or more gases that react to form the core glass are passed through the hollow glass casing, depositing a soot on the interior of the casing. The soot is converted to a glass residing upon the inner diameter of the casing, and the casing is then collapsed to form a solid preform which is drawn to an optical fiber. There is no good gaseous source for fluoride glasses, and this technique has not been extended to fabrication of fluoride glass optical preforms.
Instead, fluoride glass optical fibers are prepared by casting a central core into a previously cast casing cylinder, either in a stationary or spinning mold approach. In these techniques, a hollow glass cylinder of the casing glass is first cast. For stationary casting, only the periphery of the cylinder is hardened and the central liquid glass is allowed to drain from the mold. The core glass is then cast into the central cavity. This approach usually results in an undesirably tapered central core and a diffuse interface between the core and the casing due to melt back of the casing when the core glass is poured. The diffuse interface impairs the optical transmission of the finished optical fiber.
In the spinning mold technique, a mold is spun about its cylindrical axis and then liquid glass of the casing composition is poured into the interior. The casing glass is distributed evenly around the interior of the bore, and rapidly cooled until it solidifies as a uniform layer. The core glass is poured into the solid glass casing and permitted to solidify. This approach reduces the tapering of the core, but there is still a diffuse interface due to melt back, causing a reduction of efficiency in the internal reflection of the final optical fiber. The rate of success for this technique is only about 20 percent, with only one out of every five preforms being acceptable for use in subsequent drawing of an optical fiber.
Thus, there is a need for a technique for improved fabrication of preforms of fluoride glasses to be used in the preparation of optical fibers. Such an approach should produce uniform preforms that have high transmission of light and a sharp core/casing interface, with a high percentage of acceptable preforms. The present invention fulfills this need, and further provides related advantages.